GaN-based semiconductor element

ABSTRACT

A GaN-based semiconductor element includes a substrate, a buffer layer formed on the substrate, including an electrically conductive portion, an epitaxial layer formed on the buffer layer, and a metal structure in ohmic contact with the electrically conductive portion of the buffer layer for controlling an electric potential of the buffer layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/323,051, filed on Nov. 25, 2008, now abandoned which is acontinuation of U.S. patent application Ser. No. 12/053,841, filed onMar. 24, 2008, now abandoned which claims priority from Japanese patentapplication No. 2007-076004, filed on Mar. 23, 2007, the entire contentof which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a GaN-based semiconductor element, suchas a GaN-based heterojunction field effect transistor or the like.

2. Related Arts

A development of a field effect transistor (FET) is progressed usingmaterials for a gallium nitride (GaN) based semiconductor, especiallyusing a GaN/AlGaN based semiconductor materials, because the GaN-basedsemiconductor has a wider band gap energy than that of a semiconductorusing GaAs-based materials and has a high heat resistance as it shows asuperior performance in a high temperature.

So far, there is disclosed a GaN-based high electron mobility transistor(HEMT) comprised of a GaN-based compound semiconductor for an FET usinga GaN-based semiconductor in a published Japanese patent application No.2006-173582 (hereinafter, it is described as a document 1). In such theGaN-based HEMT, a buffer layer is formed as required. Moreover, acarrier transport layer and a carrier supply layer are epitaxial grownin order thereunto. Furthermore, electrodes are formed thereunto.

Moreover, there is disclosed a structure in a published Japanese patentapplication No. 2003-059948 (hereinafter, it is described as a document2) as another conventional technology, wherein a region of a GaN-basedsemiconductor for HEMT element is formed on a buffer layer comprising amultilayered structure including a first layer comprised of an AlN layerand a second layer comprised of a GaN layer are alternately layered on asubstrate of silicon.

So far, the GaN-based HEMT structure is used for a horizontal element asdisclosed in the above mentioned document 1 and 2. However, it isrequired to use a high resistance buffer layer, as it is necessary toreduce a leakage current flowing through the buffer layer for obtainingthe buffer layer with a high withstand voltage at the time of forming anelement for a high withstand voltage and a large current. Moreover,there are provided problems that an on-resistance is increased andconsequently an element cannot help but be broken down due to occurringa current collapse (a current slump) by being fluctuated an electricpotential of a semiconductor layer (the buffer layer) neighboring to asubstrate at the time of performing a high voltage switching in the caseof using a sapphire as an insulator for the substrate.

BRIEF SUMMARY OF THE INVENTION

The present invention has been achieved in view of the above aspects,and it is an object of the present invention to provide a semiconductorelement that can suppress the current collapse even at the time of ahigh voltage switching operation.

According to one aspect of the present invention, there is provided aGaN-based semiconductor element including a substrate, a buffer layerformed on the substrate, the buffer layer including an electricallyconductive portion, an epitaxial layer formed on the buffer layer, and ametal structure in ohmic contact with the electrically conductiveportion of the buffer layer for controlling an electric potential of thebuffer layer.

According to another aspect of the present invention, there is provideda semiconductor element, including a substrate, a buffer layer formed onthe substrate, the buffer layer including an electrically conductiveportion, an epitaxial layer formed on the buffer layer and configured togenerate a two-dimensional electron gas inside the epitaxial layer, anda metal structure. The metal structure has a first portion in directohmic contact with the electrically conductive portion of the bufferlayer, and a second portion extending outside the buffer layer and theepitaxial layer for receiving an electric potential for controlling anelectric potential of the buffer layer.

The above-noted aspects do not necessarily describe all features of allembodiments of the present invention. The present invention may alsoinclude one or more sub-combination of the features described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the invention will appearmore fully hereinafter from a consideration of the following descriptiontaken into connection with the accompanying drawing wherein one exampleis illustrated by way of example, in which;

FIG. 1 is a cross sectional view showing a schematic configuration of aGaN-based semiconductor element according to the first embodiment of thepresent invention.

FIG. 2 is a cross sectional view showing a schematic configuration of aGaN-based semiconductor element according to the second embodiment ofthe present invention.

FIG. 3 is a cross sectional view showing a schematic configuration of aGaN-based semiconductor element according to the third embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Next, every embodiment embodied the present invention will be describedin detail below, based on the drawings. Here, a duplicated descriptionis omitted with using a similar symbol for the similar component partregarding the description of every embodiment.

First Embodiment

A GaN-based semiconductor element 20 according to the first embodimentof the present invention will be described in detail below, based onFIG. 1.

The GaN-based semiconductor element 20 is to be a GaN-based highelectron mobility transistor (HEMT) as a GaN-based heterojunction fieldeffect transistor (FET).

Such the GaN-based semiconductor element 20 comprises a buffer layer 2formed on a sapphire (0001) substrate 1, a channel layer (an electrontransport layer) 3 comprised of a undoped GaN layer on the buffer layer2, and an electron supply layer 4 comprised of a undoped AlGaN layerformed on the channel layer 3. Moreover, the GaN-based semiconductorelement 20 comprises a gate electrode (G) 7, a source electrode (S) 6and a drain electrode (D) 5 formed on the electron supply layer 4respectively.

In such the GaN-based semiconductor element 20, a two-dimensionalelectron gas 8 is generated at an interface between the channel layer 3and the electron supply layer 4 because the undoped AlGaN layer (theelectron supply layer 4) is heterojunction connected to a surface of theundoped GaN layer (the channel layer 3) which is equivalent to a channellength L. Hence, the channel layer 3 becomes to be electro-conductivedue to the two-dimensional electron gas 8 becoming a carrier. Moreover,the source electrode 6 and the drain electrode 5 are comprised of Ti, analloy of Al and Si, and W to be formed as multilayer in order from thenearest region for the electron supply layer 4, that is to say, thestructure of W/Al—Si/Ti on the electron supply layer 4.

An aspect of such the GaN-based semiconductor element 20 is that thebuffer layer 2 formed on the sapphire (0001) substrate 1 is comprised ofthe electro-conductive semiconductor layer and the GaN-basedsemiconductor element 20 includes the structure to be able to controlthe electric potential of such the electro-conductive semiconductorlayer (the buffer layer 2).

According to the present embodiment, the buffer layer 2 is comprised ofan n-GaN layer having n-type conductivity. Moreover, a configuration isadopted according to the present embodiment, wherein the S 6 isimplanted into epitaxial layers (the channel layer 3 and the electronsupply layer 4) formed on the buffer layer 2 to be extended to a depthreaching the buffer layer 2 for ohmic contacting to the buffer layer 2.

It is possible to manufacture as follows regarding the GaN-basedsemiconductor element 20 comprising the above mentioned configuration.Here, a metal organic chemical vapor deposition (MOCVD) equipment isused for a growth system and the sapphire (0001) substrate 1 is used fora substrate.

1. First, the sapphire (0001) substrate 1 is to be introduced into achamber of the MOCVD equipment, and the substrate is to be heated up toa temperature of 600° C. at a degree of vacuum of 100 hPa aftervacuuming the chamber as not higher than 1×10⁻⁶ hPa using a turbomolecular pump (TMP). After stabilizing the temperature thereof, agrowth of the buffer layer 2 comprised of a GaN layer is to be performedby introducing raw materials of a trimethylaluminum (TMA) with a flowrate of 100 cm³/min and an ammonia with the flow rate of 12 l/minrespectively, onto the surface of the substrate 1 with rotating thesubstrate 1 as 900 rpm. Hence, the buffer layer 2 of approximately 50 nmthickness is able to be grown after a growth time of four minutes. Thus,the buffer layer 2 comprised of an n-GaN layer is formed, by doping animpurity of n-type into the buffer layer 2 comprised of the GaN layerthereafter.

2. Next, a growth of the channel layer (the electron transport layer) 3is to be performed by introducing a trimethylgallium (TMG) with the flowrate of 300 cm³/min and the ammonia with the flow rate of 12 l/min ontothe buffer layer 2, after heating up the substrate 1 with the ammonia asthe flow rate of 12 l/min to be held at the substrate temperature of1050° C. Hence, the channel layer 3 of approximately 3000 nm thicknessis able to be grown after a growth time of 2000 seconds.

3. Next, a growth of the electron supply layer 4 comprised of anAl_(0.25)Ga_(0.75)N layer is to be performed by introducing the TMA withthe flow rate of 50 cm³/min, the TMG with the flow rate of 100 cm³/minand the ammonia with the flow rate of 12 l/min onto the channel layer 3.Hence, the electron supply layer 4 of approximately 20 nm thickness isable to be grown after a growth time of 40 seconds. Thus, it becomesable to complete the layered structure shown in FIG. 1.

4. Next, an element separation is to be performed, using such as achlorine-based gas or the like.

5. The drain electrode 5 is to be formed thereafter, by forming a SiO₂insulating film layer, by opening parts for forming the source electrode6 and the drain electrode 5 to expose a surface of the electron supplylayer 4 by performing a patterning process with masking a part forforming the gate electrode 7 using the patterned SiO₂ film layer, and bydepositing Ti, the alloy of Al and Si, and W in order thereunto.

6. The part for forming the source electrode 6 is to be removed byperforming an etching process to the depth reaching the buffer layer 2comprised of the n-GaN layer thereafter. And, the source electrode 6 isto be formed to the depth reaching the buffer layer 2 for ohmiccontacting thereto, by depositing Ti, the alloy of Al and Si, and W inorder inside a ditch formed by removing the part at the etching process.

7. Next, the gate electrode 7 is to be formed, by removing the abovementioned SiO₂ film layer for masking the part for forming the gateelectrode 7, by forming another SiO₂ film layer for masking the sourceelectrode 6 and the drain electrode 5 with a open part for forming thegate electrode 7, and by depositing Ni and Au thereunto. As a result, itbecomes able to manufacture the GaN-based semiconductor element(GaN-based HEMT) as shown in FIG. 1.

According to the first embodiment including the above mentionedstructure, the following functions and advantages are able to beobtained.

It is able to fix the electric potential of the buffer layer 2 comprisedof the n-GaN layer for being equal to the electric potential of thesource electrode 6 because the source electrode 6 is ohmic contacted tothe buffer layer 2 (the electro-conductive semiconductor) comprised ofthe n-GaN layer having n-type conductivity. That is to say, it becomesable to control the electric potential of the buffer layer 2 comprisedof the n-GaN layer to be a predetermined electric potential by using thesource electrode 6.

Therefore, it is able to suppress the current collapse (the currentslump) occurred by being fluctuated the electric potential of the bufferlayer 2 neighboring to the substrate 1 at the time of performing thehigh voltage switching. Hence, it becomes able to prevent theon-resistance from being increased and the element from cannot help butbe consequently broken down due to occurring the current collapse.

It becomes able to realize the GaN-based semiconductor element 20 to beable to suppress the current collapse even at the time of performing thehigh voltage switching.

It is able to control the electric potential of the buffer layer 2comprised of the n-GaN layer having n-type conductivity to be apredetermined electric potential by using the source electrode 6. Hence,it becomes able to suppress the current collapse occurred by beingfluctuated the electric potential of the buffer layer 2 neighboring tothe substrate 1 at the time of performing the high voltage switching.

Regarding the GaN-based heterojunction field effect transistor, thestrongest electric field is applied to between the gate electrode 7 andthe drain electrode 5 at the time of turning off, and the electric fieldflocks to around the edge of the gate electrode 7 at the drain electrode5 side. However, according to the present embodiment, it becomes able torelax the flock of the electric field by controlling (fixing) theelectric potential of the buffer layer 2 comprised of the n-GaN layerhaving n-type conductivity (to be a predetermined electric potential).

It is able to enhance the withstand voltage between the buffer layer 2and the drain electrode 5 by forming the channel layer 3 comprised ofthe n-GaN layer to be the thickness of not less than 3000 nm. Thus, itbecomes able to realize the GaN-based semiconductor element 20 for thehigh withstand voltage and the large current, having a high reliabilitywith the high withstand voltage at the time of operating thereof.

Second Embodiment

Next, a GaN-based semiconductor element 20A according to the secondembodiment of the present invention will be described in detail below,based on FIG. 2.

An aspect of such the GaN-based semiconductor element 20A is that thefollowing structure is to be adopted as a structure to be able tocontrol an electric potential of a buffer layer 2 comprised of an n-GaNlayer.

That is to say, in the structure to be able to control the electricpotential of the buffer layer 2, there is provided a metal layer 9,which is selectively formed at a part except a region for forming thebuffer layer 2 on a sapphire (0001) substrate 1, and which is ohmiccontacted to the buffer layer 2. Moreover, the structure includes aconfiguration that a source electrode 6 is implanted into epitaxiallayers (a channel layer 3 and an electron supply layer 4) formed on thebuffer layer 2 and the metal layer 9, and that the source electrode 6 isextended to a depth reaching the metal layer 9 for electricallycontacting thereto.

Such the metal layer 9 is to be formed using any of high melting metalmaterials, such as Ni, Nb, Mo, Ta, W, Re, Os, Ir, Pt, or the like.

It is possible to manufacture as follows regarding the GaN-basedsemiconductor element 20A comprising the above mentioned configuration.

1a. First, the metal layer 9 comprised of any of the high melting metalmaterials, such as Ni, Nb, Mo, Ta, W, Re, Os, Ir, Pt, or the like, is tobe selectively formed at the part except the region for forming thebuffer layer 2 on the sapphire (0001) substrate 1, before being thebuffer layer 2 grown at the above mentioned process 1 described in thefirst embodiment. At the time thereof, it is preferable to form themetal layer 9 as thicker than the thickness of the buffer layer 2.

2a. The above mentioned processes 1 to 5 are to be performed thereafter.

3a. A part for forming the source electrode 6 is to be removed byperforming an etching process to the depth reaching the buffer layer 2comprised of the n-GaN layer thereafter. And, the source electrode 6 isto be formed to the depth reaching the buffer layer 2 for ohmiccontacting thereto, by depositing Ti, an alloy of Al and Si, and W inorder inside the ditch formed by removing the part at the etchingprocess. Moreover, an upper part of the source electrode 6 is to beexposed from a top surface of the electron supply layer 4 at the timethereof.

4a. Next, a gate electrode 7 is to be formed by performing the abovementioned process 7. As a result, it becomes able to manufacture theGaN-based semiconductor element 20A (the GaN-based HEMT) as shown inFIG. 2.

According to the second embodiment including the above mentionedstructure, the following functions and advantages are able to beobtained.

It is able to control (fix) the electric potential of the buffer layer 2comprised of the n-GaN layer (the electro-conductive semiconductorlayer) to be a predetermined electric potential by using the sourceelectrode 6 and the metal layer 9. Hence, it becomes able to suppressthe current collapse occurred by being fluctuated the electric potentialof the buffer layer 2 neighboring to the substrate 1 at the time ofperforming the high voltage switching.

It becomes able to obtain the excellent ohmic contact by forming themetal layer 9 using any of the high melting metal materials, such as Ni,Nb, Mo, Ta, W, Re, Os, Ir, Pt, or the like.

Third Embodiment

Next, a GaN-based semiconductor element 20B according to the thirdembodiment of the present invention will be described in detail below,based on FIG. 3.

An aspect of such the GaN-based semiconductor element 20B is that thefollowing structure is to be adopted as a structure to be able tocontrol an electric potential of a buffer layer 2 comprised of an n-GaNlayer.

That is to say, in the structure to be able to control the electricpotential of the buffer layer 2, there is provided a metal layer 9,which is selectively formed on a part except a region for forming thebuffer layer 2 on a sapphire (0001) substrate 1, and which is ohmiccontacted to the buffer layer 2. Moreover, the structure includes aconfiguration that a mesa structure 11 is formed at a part of epitaxiallayers (a channel layer 3 and an electron supply layer 4) formed on thebuffer layer 2 to be deep for reaching the metal layer 9, and that anelectrode 10 is formed on a part of a top surface of the metal layer 9exposed by opening the part of the top surface of the metal layer 9.

It is possible to manufacture as follows regarding the GaN-basedsemiconductor element 20B comprising the above mentioned configuration.

1b. First, the metal layer 9 comprised of any of high melting metalmaterials, such as Ni, Nb, Mo, Ta, W, Re, Os, Ir, Pt, or the like, is tobe selectively formed on the part of the region wherein the buffer layer2 is formed on the sapphire (0001) substrate 1, before being the bufferlayer 2 grown at the above mentioned process 1 described in the firstembodiment.

2b. The above mentioned processes 1 to 4 are to be performed thereafter.

3b. A source electrode 6 and a drain electrode 5 are to be formedthereafter, by forming a SiO₂ insulating film layer, by opening partsfor forming the source electrode 6 and the drain electrode 5 with beingthe surface of the electron supply layer 4 exposed by performing apatterning process with masking a part for forming a gate electrode 7using the patterned SiO2 film layer, and by depositing Ti, an alloy ofAl and Si, and W in order thereunto.

4b. Next, the gate electrode 7 is to be formed by performing the abovementioned process 7.

5b. Next, the mesa structure 11 is to be formed to be deep for reachingthe metal layer 9 at the part of the epitaxial layers (the channel layer3 and the electron supply layer 4) formed on the buffer layer 2, byetching (dry etching) using such as a chlorine-based inductively coupledplasma (ICP) method or the like. Hence, the part of the metal layer 9 isto be opened and exposed thereby.

6b. Next, the electrode 10 is to be formed using a deposition method ona part of the metal layer 9 exposed by the dry etching. As a result, itbecomes able to manufacture the GaN-based semiconductor element 20B (theGaN-based HEMT) as shown in FIG. 3.

According to the third embodiment including the above mentionedstructure, the following functions and advantages are able to beobtained.

It is able to control (fix) the electric potential of the buffer layer 2comprised of the n-GaN layer (the electro-conductive semiconductorlayer) to be a predetermined electric potential by using the electrode10 formed on the part of the metal layer 9 and the metal layer 9 to beohmic contacted to the buffer layer 2. Hence, it becomes able tosuppress the current collapse occurred by being fluctuated the electricpotential of the buffer layer 2 neighboring to the substrate 1 at thetime of performing the high voltage switching.

Moreover, it is also able to embody with modifying as follows regardingthe present invention.

According to the above described every embodiment, the example is shownregarding the GaN-based heterojunction FET (GaN-based HEMT). However,the present invention is not limited to the GaN-based HEMT, as it isalso applicable to a GaN-based semiconductor element, such as an MOSFET,a diode, a bipolar transistor, or the like, with using a GaN layer.

Moreover, the present invention is also applicable to a GaN-basedsemiconductor element, with using such as a SiC substrate, a Sisubstrate, a GaN substrate, a MgO substrate, a ZnO substrate, or thelike, instead of the sapphire substrate.

According to the above described every embodiment, there is described asone example regarding the configuration that the buffer layer 2 iscomprised of the n-GaN layer having n-type conductivity. However, thepresent invention is also applicable to a GaN-based semiconductorelement comprised with using a buffer layer comprised of a layeredstructure of an AlN layer and a GaN layer and of an n-GaN layer havingn-type conductivity formed on such the layered structure, or with usinga buffer layer comprised of an n-AlN layer having n-type conductivity.

According to the above described first embodiment, the configuration isadopted, wherein the source electrode 6 is implanted into the epitaxiallayers formed on the buffer layer 2 to be extended to the depth reachingthe buffer layer 2 for ohmic contacting to the buffer layer 2. However,the present invention is not limited to such the configuration, as it isalso applicable to a GaN-based semiconductor element comprising aconfiguration that another part of any of layers, which is comprised ofany of metal materials and which is provided with the source electrode6, is implanted into the epitaxial layers formed on the buffer layer 2to be extended to the depth reaching the buffer layer 2 for ohmiccontacting to the buffer layer 2.

Similarly, regarding the above described second embodiment, the presentinvention is also applicable to a GaN-based semiconductor elementcomprising a configuration that another part of any of layers, which iscomprised of any of metal materials and which is provided with thesource electrode 6, is implanted into the epitaxial layers formed on thebuffer layer 2 to be extended to the depth reaching the metal layer 9for electrically contacting to the metal layer 9.

Furthermore, it is also able to obtain the similar functions andadvantages by the configuration excluding the electrode 10 according tothe above described third embodiment as shown in FIG. 3.

The present invention is not limited to the above described embodimentsand various; further modifications may be possible without departingfrom the scope of the present invention.

This application is based on the published Japanese patent applicationNo. 2006-173582 filed on Nov. 14, 2005 and the published Japanese patentapplication No. 2003-059948 filed on Aug. 20, 2001, entire content ofwhich is expressly incorporated by reference herein.

1. A GaN-based semiconductor element, comprising: a substrate; a bufferlayer formed on the substrate, the buffer layer including anelectrically conductive portion; an epitaxial layer formed on the bufferlayer; and a metal structure in ohmic contact with the electricallyconductive portion of the buffer layer for controlling an electricpotential of the buffer layer; wherein the metal structure includes anelectrode implanted into the epitaxial layer to be extended to a depthreaching the electrically conductive portion of the buffer layer forohmic contact with the electrically conductive portion of the bufferlayer.
 2. The GaN-based semiconductor element according to claim 1,further comprising a source electrode, a gate electrode, and a drainelectrode formed on the epitaxial layer; wherein the electrode is thesource electrode.
 3. The GaN-based semiconductor element according toclaim 1, wherein the metal structure includes a metal layer in ohmiccontact with the electrically conductive portion of the buffer layer ata part of a region where the buffer layer is formed on the substrate,and an electrode implanted into the epitaxial layer to be extended to adepth reaching the metal layer for electrical contact with the metallayer.
 4. The GaN-based semiconductor element according to claim 3,further comprising a source electrode, a gate electrode, and a drainelectrode formed on the epitaxial layer, wherein the electrode is thesource electrode.
 5. The GaN-based semiconductor element according toclaim 3, wherein the metal layer is formed of at least one selected fromthe group consisting of Ni, Nb, Mo, Ta, W, Re, Os, Ir, and Pt.
 6. AGaN-based semiconductor element, comprising: a substrate; a buffer layerformed on the substrate, the buffer layer including an electricallyconductive portion; an epitaxial layer formed on the buffer layer; and ametal structure in ohmic contact with the electrically conductiveportion of the buffer layer for controlling an electric potential of thebuffer layer; wherein the metal structure includes: a metal layer inohmic contact with the electrically conductive portion of the bufferlayer at a part of a region where the buffer layer is formed on thesubstrate, wherein a deep mesa structure is formed in a part of theepitaxial layer to expose a portion of the metal layer; and an electrodeformed on the exposed portion of the metal layer for electrical contactwith the metal layer.
 7. The GaN-based semiconductor element accordingto claim 6, wherein the metal layer is formed of at least one selectedfrom the group consisting of Ni, Nb, Mo, Ta, W, Re, Os, Ir, and Pt.
 8. AGaN-based semiconductor element, comprising: a substrate; a buffer layerformed on the substrate, the buffer layer including an electricallyconductive portion; an epitaxial layer formed on the buffer layer; and ametal structure in ohmic contact with the electrically conductiveportion of the buffer layer for controlling an electric potential of thebuffer layer; wherein the epitaxial layer comprises: a channel layer ofundoped GaN layer formed on the buffer layer; an electron supply layerof AlGaN formed on the channel layer; and a gate electrode, a sourceelectrode and a drain electrode formed on the electron supply layerrespectively, to define a GaN-based heterojunction field effecttransistor structure.
 9. A GaN-based semiconductor element, comprising:a substrate; a buffer layer formed on the substrate, the buffer layerincluding an electrically conductive portion; an epitaxial layer formedon the buffer layer; and a metal structure in ohmic contact with theelectrically conductive portion of the buffer layer for controlling anelectric potential of the buffer layer; wherein the epitaxial layerincludes at least two GaN-based layers with a heterojunctiontherebetween.
 10. A semiconductor element, comprising: a substrate; abuffer layer formed on the substrate, the buffer layer including anelectrically conductive portion; an epitaxial layer formed on the bufferlayer and configured to generate a two-dimensional electron gas insidethe epitaxial layer; and a metal structure having a first portion indirect ohmic contact with the electrically conductive portion of thebuffer layer, and a second portion extending outside the buffer layerand the epitaxial layer for receiving an electric potential forcontrolling an electric potential of the buffer layer.
 11. Thesemiconductor element according to claim 10, wherein the second portionis implanted into the epitaxial layer to be extended to a depth reachingthe electrically conductive portion of the buffer layer to make thefirst portion ohmic contact with the electrically conductive portion ofthe buffer layer.
 12. The semiconductor element according to claim 11,further comprising a source electrode, a gate electrode, and a drainelectrode formed on the epitaxial layer, wherein the second portion isthe source electrode.
 13. The semiconductor element according to claim11, wherein the first portion includes a metal layer formed on thesubstrate, in direct ohmic contact with the electrically conductiveportion of the buffer layer, and at least partially co-elevational withthe buffer layer.
 14. The semiconductor element according to claim 10,wherein the first portion includes a metal layer formed on thesubstrate, in direct ohmic contact with the electrically conductiveportion of the buffer layer, and at least partially co-elevational withthe buffer layer, and a mesa structure is formed in a part of theepitaxial layer to expose a portion of the metal layer; and the secondportion includes an electrode formed on the exposed portion of the metallayer.
 15. The semiconductor element according to claim 13, wherein themetal layer is thicker than the buffer layer.
 16. The semiconductorelement according to claim 14, wherein the electrode is co-elevationalwith the epitaxial layer.
 17. The semiconductor element according toclaim 10, wherein the buffer layer includes at least one selected fromthe group consisting of a buffer layer of n-GaN, a buffer layer of (i) alayered structure of AlN and GaN and (ii) an n-GaN layer formed on thelayered structure, and a buffer layer of n-AlN.
 18. The semiconductorelement according to claim 10, wherein the epitaxial layer includes achannel layer of undoped GaN formed on the buffer layer; and an electronsupply layer of AlGaN formed on the channel layer; wherein thetwo-dimensional electron gas is generated in the channel layer near aninterface between the electron supply layer and the channel layer. 19.The semiconductor element according to claim 10, wherein the epitaxiallayer includes GaN-based semiconductor materials.